Thermoset resins such as epoxy resins are commonly used, for example, in the microelectronic and aircraft industries. The regulations of both the microelectronic and aircraft industries mandate the use of flame retardant epoxy resins. Typically, flame retardant epoxy resins include epoxy resins containing bromine. However, increasing the flame retardance of an epoxy resin, by brominating an epoxy resin, reduces its fracture resistance, making it difficult to process these materials into finished products.
For example, a major problem in the electronics industry is the drillability of brominated epoxy composites used in printed circuit board fabrication, because brominated epoxy composites are known to be very brittle. Thus, while bromination of epoxy compositions is necessary to improve the flame resistance of the epoxies, particularly when such epoxies are to be used in microelectronic applications, brominated epoxy resin materials used in microelectronic applications easily shatter during drilling procedures and, in turn, limit their applications.
Other known flame retardant epoxy resins useful in electrical laminates applications are non-brominated epoxy resins, for example, phosphorous-containing epoxy resins such as those described in U.S. Pat. No. 6,403,220; U.S. Patent Application Publication No. 2002/0119317 A1; and PCT Publication No. WO 99/00451. These bromine-free epoxy resins also have the disadvantage of being brittle and may be difficult to drill during printed circuit board fabrication.
Epoxies in general are known to be very difficult to toughen and some epoxies are too brittle to toughen effectively. Moreover, increasing the fracture toughness of brittle epoxies often comes at the expense of modulus and use temperature, creating unacceptable limits on the applicability of these resins.
Recent efforts have concentrated on using block copolymer self-assembly to toughen epoxies with a minimal impact on the glass transition temperature and modulus and with the advantages of simple processing and low cost. For example, Dean, J. M.; Lipic, P. M.; Grubbs, R. B.; Cook, R. F.; Bates, F. S. J., “Micellar Structure and Mechanical Properties of Block Copolymer-Modified Epoxies,” J. Polym. Sci. Part B Polymer Physics, 2001, 39, 2996-3010, discloses that block copolymers self-assembled into vesicles and spherical micelles can significantly increase the fracture resistance of model bisphenol A epoxies cured with a tetrafunctional aromatic amine curing agent; a general correlation between the ratio of the separation between particles and the particle diameter appears to conform to classical toughening mechanisms. These morphologies share the same basic spherical shape, but the larger dimensions of the vesicles produced up to a three-fold increase in the fracture resistance.
It would be desirable to find another way to further increase fracture resistance, and flame retardance of epoxy resins without sacrificing other properties of the epoxy resin such as use temperature and modulus.
It is desired to provide a composition with an improved fracture resistance, which overcomes the disadvantages of known materials, particularly which overcomes the drilling problems of prior materials.